Paint sludges arise usually in paint spray installations in which, in a chamber, a suspended article such as a vehicle body, is subjected to spraying with a paint. The excess of the spray can reach walls of the spray booth as a so-called overspray and the overspray is washed with water away from the spray booth to form a contaminated aqueous suspension and solution which is led to a settling tank or basin. As part of the washing of the walls from the overspray and the removal of the overspray from the spray booth, a de-adhesive action is required. That means that additives must be provided for the scrubbing water which will cause the breakdown of the film forming paint spray into discrete particles which, depending upon the type of additive, will either float or settle out. In the mass production of automobiles, the paint tunnels or booths are operated at high rates and the de-adhesive substances which are used are primarily bentonite or clay minerals, waxes in certain cases and the like. Other customary additives are defoamers and sedimentation aides or auxiliaries, and biocides.
Especially in the case of so-called water paints, there is a high organic loading of the washing liquor with respect to solvents. Additional organic matter is imported in the form of the defoamers, sedimentation auxiliaries and biocides.
Because of the accumulation of organic loading, the water of the recycled scrubbing liquor must be replaced at time intervals of 4 to 12 months. The paint sludge can be dewatered in filter presses to a dry content of about 50 weight percent and, in the past, was disposed of in landfills or incinerated.
Since landfill disposal of paint sludges is no longer environmentally sound and in many places is illegal and therefore impossible, incineration is the only prior art approach which could be considered up to this point in time.
However, this method of disposal poses problems as well since, by and large, there is insufficient incineration capacity and incineration does not allow any kind of recycling or recovery of usable materials from the waste.
A variety of different recycling approaches have been attempted without success. For example, efforts have been made to recycle the paint sludge as an additive to liquors with reduced quality requirements as fillers. Efforts have also been made to work paint sludge into sealing compositions or bituminous products, to substitute the paint sludge for phenolic resins in the production of plastic molded products and to recover titanium dioxide from the paint sludge by a combustion process.
All of these processes have limitations. All of the described techniques with the exception of the recovery of titanium dioxide are dependent upon conditions of the paint sludge such as age, level of impurities, biological contamination and the presence of toxic or noxious components so that usually only a small portion of the paint sludge which is generated can be used.
In the production of titanium dioxide, combustion can create an ecologic problem and, furthermore, the technique is usually economically disadvantageous.